i) Field of Invention
The present invention relates to the mechanical refining of wood chips to produce wood pulp and more particularly, to a method of producing uniform and superior quality pulp from wood chips with disc refiners employing low intensity refining.
ii) Description of the Prior Art
In recent years there has been a considerable amount of both theoretical and experimental research into the factors governing fibre residence time in wood chip refiners. This has led to the concept of refining intensity, defined as the specific energy delivered per bar impact. The constitutive equations for material flows within refiners developed by Miles and May (J. Pulp Paper Sci. 16(2): J63(1990) and Paperi ja Puu 73(9):852(1991) can be used to calculate pulp velocity and the corresponding residence time of the pulp in the refiner. The residence time determines the specific energy per bar impact which Miles and May defined as refining intensity. Currently, many developments in mechanical pulping are focussed on reducing energy consumption while maintaining pulp quality at a desirable level. The refining intensity, which is a deciding factor for the quality of pulp produced at a given total specific energy, is itself highly dependent on the rotational speed of the refiner. In current commercial systems, the rotational speed of the refiner is determined by the frequency of the electrical supply, so that double disc refiners run at either 1200 or 1500 RPM and single disc refiners at 1500 or 1800 RPM. Therefore, regardless of the process requirements, the refining intensity is fixed by the system design and local generating frequency.
Jones et al (U.S. Pat. No. 3,411,720) disclose that mechanical pulp with greater strength could be produced by devising a refining process in which the amount of energy absorbed by the wood material is increased. They achieve this by increasing the residence time of the pulp in the refiner using a combination of sinuous plates or tapered plates and control rings on the periphery of the plates. Although they specified relative speeds of between 900 and 3600 RPM the process was never proven to be practical or effective.
Other methods to change the pulp residence time and the applied energy by changing the steam flow within zones in a refiner, have been reported. U.S. Pat. No. 5,335,865 discloses removing some of the steam from a first refining zone before it enters a second zone, or from the first two refining zones before entering a third (U.S. Pat. No. 5,248,099). These methods are based on the mistaken belief that steam and pulp velocities are equal. Steam flow has little effect on either average pulp velocity or residence time as calculated by Miles, Paperi ja Puu 73 (9):852(1991).
U.S. Pat. No. 2,035,994 discloses using variable disc speed to control pulp throughput using a stock refiner. A stock refiner is different from a mechanical disc refiner in that the stock refiner is employed to refine pulp while a mechanical disc refiner is used to refine wood chips. The process operates at low consistencies (3 to 6%) and rotational speeds (300 to 900 RPM). While these conditions are suitable for small modifications and improvements to pulp fibres before papermaking they are not suitable for refining wood chips into pulp fibres.
In an effort to circumvent the limitations imposed by electrical frequency and to reduce specific energy consumption in a single disc refiner, one equipment supplier has advocated an increase in refining intensity by employing a gearbox to raise the rotational speed to 2300 RPM. Similarly, in a double disc refiner (U.S. Pat. No. 5,167,373), an increase in feed-end speed from 1200 to 1800 RPM (U.S. Pat. No. 5,167,373) is advocated. However, these approaches do not account for situations where a fixed high level of refining intensity may not be appropriate for different raw materials and end product requirements.
Operating at higher than conventional disc speeds has been most effective when the first refining stage was operated at high refining intensity and the second stage was operated conventionally. (Tappi Journal 74(3):221(1991) and J. of Pulp Paper Sci. 19(1):J12(1993)). The optimum energy saving at desired pulp and fibre properties was obtained by employing a smaller portion of the total specific energy in the first, high intensity stage. A typical split in specific energy between the first and second stages of refining would be 40/60. Increasing further the refining intensity or the proportion of the specific energy applied in the first stage lowers the total energy required to reach a given freeness. However, it also lowers the average fibre length and pulp strength, limiting the advantage of the energy savings that was reported.
U.S. Pat. No. 5,540,392, issued to Broderick et al. discloses that it is possible to reduce energy by up to 18% in a two-stage refining system. At least 65% of the total energy is applied in a low intensity first stage refiner operating at conventional disc rotational speeds. Low refining intensity was achieved by raising the consistency from 26 to 30%. The remaining energy is applied in a high intensity second stage refiner. The pulp properties are reported to be at least as good as or better than that produced by conventional refiners although all their examples are for highly sulphonated pulps.
There is no known approach in the prior art that addresses the need to reduce refining intensity below conventional levels in order to improve the quality of wood fibres.
It is an object of this invention to provide a method for mechanically refining wood chips to produce wood pulp exploiting low refining intensity achieved by rotating impact members which are rotated at speeds lower than conventional speeds.
It is a further object of the invention to provide an improvement to the conventional methods for refining wood chips into pulp or individual pulp fibres by mechanical means in a single or double disc refiner. More specifically, the improvement comprises increasing pulp quality by reducing refining intensity in one or more specific refining stages.
It is a particular object of the present invention to control refining intensity in a single refining stage employing low refining intensity.
Another object of the present invention is to control refining intensity in two distinct refining stages employing low refining intensity at least in the second stage of refining.
It is a further object of this invention to control the refining intensity in at least two or more distinct refining stages employing low intensity in at least two of the refining stages, one of the stages being the last.
It is yet another object of the present invention to control refining intensity in two or more distinct refining stages employing low refining intensity in a pre-refining stage and a final refining stage.
It is yet a further object of the invention to control refining intensity in three distinct refining stages employing low refining intensity in each stage.
In a further object of this invention low refining intensity is employed in refining rejects in a single or multiple stage refining operation.
It is yet another object of this invention to provide a method of refining wood chips to wood fibre pulp with monitoring of a fibre quality parameter.
The objects of this invention, to employ low refining intensity in a distinct refining stage or stages can be carried out by rotating the discs of a double disc refiner or the disc of a single disc refiner at rotational speeds that are lower than that conventionally employed.
In accordance with the invention there is provided in a method for mechanical refining of a wood chip composition to produce wood pulp in which the wood chip composition is subjected to at least one refining stage comprising transfer of energy to the wood chip composition under impact of rotating impact members, the improvement in which at least a final stage of said at least one refining stage is carried out in a refiner selected from a double disc refiner or a single disc refiner at a low refining intensity in which the rotating impact members of the double disc refiner rotate at less than 1200 RPM and the rotating impact members of the single disc refiner rotate at less than 1500 RPM.
In accordance with another aspect of the invention there is provided in an apparatus for refining wood chips to wood fibre pulp comprising at least one refiner having a rotatable impact member, a monitor to detect a parameter of the wood fibre pulp and control means to adjust operation of the refiner in response to the detected parameter, to establish a predetermined value of the parameter, the improvement wherein said apparatus additionally includes a motor speed controller effective to control the rate of rotation of the rotatable impact member to vary the intensity of refining in the refiner, in response to a fibre quality parameter detected by said monitor.
In accordance with still another aspect of the invention there is provided a method of refining wood chips to wood fibre pulp comprising: refining wood chips in at lest one refiner having at least one rotatable impact member to produce a product wood fibre pulp, monitoring the product wood fibre pulp to detect a fibre quality parameter of the pulp, and altering the rate of rotation of the at least one rotatable impact member to vary the intensity of refining in the at least one refiner, in response to the fibre quality parameter detected, to establish a predetermined value of the fibre quality parameter in the product wood fibre pulp.
The invention provides a refining process capable of improving pulp fibre quality, to the likeness of kraft fibre, by low intensity refining enabled by a speed reduction of the disc or discs in a refiner. This unexpected finding was first discovered while employing low intensity refining at disc speeds of less than 1200 RPM in a single stage refiner.
Subsequently, it has been found that the improvement of pulp-fibre quality is also evident in refining systems employing distinct multiple refining stages provided that at least the final refining stage is carried out at low intensity enabled by a reduction of disc speed to below conventional speeds. It has also been discovered that fibre quality was improved in a multi stage refining system when each refiner in a multiple series was operated at low refining intensity enabled by a reduction of disc speed to less than conventional operating speeds.
Double disc refiners conventionally operate at disc rotations of 1200 RPM or more. In the present invention, when employing a double disc refiner or a plurality of double disc refiners in sequence, the discs rotate at less than 1200 RPM, preferably not more than 1150 RPM, more preferably 850 to 1000 RPM and most preferably about 900 RPM to provide the desired rotation of the impact members which are supported on the rotating discs.
Single disc refiners conventionally operate at a disc rotation of 1500 RPM or more, and typically at 1500 RPM or 1800 RPM. In the present invention, when employing a single disc refiner or a plurality of single disc refiners in sequence, the disc rotates at less than 1500 RPM, preferably not more than 1450 RPM, more preferably at 1100 to 1300 RPM and most preferably at about 1200 RPM.
In practice it is most expedient at present to employ speeds which are multiples of 300 RPM, because equipment manufacturers market disc refiners with motors which provide operational speeds which are multiples of 300 RPM; for example, 900 RPM, 1200 RPM, 1500 RPM and 1800 RPM. In general operating at speeds which are not multiples of 300 RPM requires additional equipment permitting variation of the output of the motor in the commercially available disc refiners.
In general the refining of wood chips to wood pulp comprises breaking the wood chips down into the component fibres, followed by fibre development in which the fibres are fatigued to render them flexible and collapsible.
This invention can also be implemented by applying a constant low intensity, low energy treatment in a pre-refiner by utilizing a fixed rotational speed of less than 900 RPM and preferably at 600 RPM.
The pre-refiner provides a preliminary gentle refining prior to the main refining stage or stages.
In another embodiment of this invention a rejects refiner is operated at low intensity enabled by a reduction in disc speed to less than 1200 RPM to yield pulp fibre of extremely high quality. The rejects are, in particular, screen rejects from a wood chip pulp of the mainline refiners.
All or any combination of the above processes can be implemented to maximize the quality of the pulp fibres to a level heretofore non-existent commercially.
A preferred embodiment applied to existing commercial TMP (thermo-mechanical pulp) refiner installations employing double disc refiners would be the operation of the secondary and/or tertiary refiner in such an installation at low intensity enabled by a reduction in disc speed to less than 1200 RPM. In the commercial installation the refining intensity is too high for the fibres at that point in their development. It has been discovered that by employing low refining intensity according to this invention the secondary and/or tertiary refiner can be fully loaded when required.
In each of the above cases low intensity refining is enabled by a speed reduction of the disc refiner which typically is achieved by known means in the art such as a variable frequency AC drive, a gear box or a simple set speed motor design, whichever is most appropriate.
The present invention is not necessarily directed to saving refining energy per se, although some energy saving can be achieved. The invention has major application in that it may be used to direct energy towards fibre quality development rather than fibre damage, as is the case with conventional refining systems.
In an apparatus embodiment of the present invention employing a double disc refiner one or both discs is rotated at less than 1200 RPM and preferably at 900 RPM in order to achieve a desired low refining intensity required to direct the applied energy towards fibre quality development.
In another apparatus embodiment of this invention employing a single disc refiner the disc is rotated at less than the conventional 1800 RPM or 1500 RPM, and more especially at less than 1500 RPM.
Thus, there is provided in the present invention a variety of novel features which can afford significantly greater on-line control of the refining process to produce quality fibre.
Additionally, pulp quality control can be greatly enhanced through feedback adjustments of motor rotational speed, i.e., refining intensity. Indeed this will provide another variable, besides specific energy, that can be manipulated to control pulp properties. This will give the possibility to independently control on-line two of the pulp properties such as freeness and average fibre length which is an advantage over conventional control where only one variable, specific energy, is adjusted to control only one pulp property such as freeness.
In a typical refiner installation a device is employed which monitors pulp quality by measuring fibre length and specific surface or freeness and maintains a set quality standard by motor load, for example, by plate gap adjustment. If, now, for example, with the present invention the fibre length is sensed as being too low, a signal is sent to the motor speed controller of the refiner, to reduce RPM thereby lowering refining intensity and thus restoring fibre length to a preset value. Such monitoring devices are commercially available. Other fibre quality parameters may be sensed in the same way, for example, flexibility or coarseness and a signal sent to the motor speed controller to change the RPM to restore the quality parameter to a desired level.